External skeletal fixator systems are used on fractured bones or joints during osteosynthesis typically for more serious injuries involving multiple or compound fractures. Pins are drilled through a patient's skin and into a fractured bone. Clamps secure the pins to a common connecting rod, creating a rigid frame that immobilizes the bone.
The Kirschner-Ehmer external fixator system is commonly used in veterinary orthopedic surgery. The system includes: a standard connecting rod; clamps adapted to slide over the ends of the connecting rod; fixator pins; and bolts for tightening the clamps around the connecting bars, which bolts have through-holes near the head for receiving the fixator pins' shafts.
The Kirschner-Ehmer system precludes installation of an additional clamp between two installed clamps on the connecting bar or removal of an intermediate clamp without disassembly of the entire connecting rod. This is because the Kirschner-Ehmer clamp includes a narrow slot leading to a wider channel, into which channel a connecting rod is inserted axially. The connecting rod will not fit into the slot to allow for transverse mounting. Consequently, the surgeon must anticipate the number of clamps required and slide them onto the connecting rod before insertion of the end pins into the bone. This is especially limiting if an additional pin or clamp is required at the completion of surgery or at subsequent patient visits. In addition, the Kirschner-Ehmer clamp undergoes plastic deformation when tightened, permanently deforming and fatiguing the material. Consequently, reuse of the clamp is discouraged. Furthermore, the Kirschner-Ehmer clamp is inapplicable in ring fixators such as the Ilizarov external ring fixator.
A fixator clamp produced by Synthes.RTM. permits transverse installation of a clamp on a connecting rod between two installed clamps without rod disassembly. It includes a slot for receiving a connecting rod, a hole for receiving a pin in an orthogonal direction relative to the connecting rod, a clevis-shaped clamp, and a bolt and nut which simultaneously secure the pin in the hole and the clamp on the connecting rod. This design is limited to orthogonal pin placements relative to the connecting rod and is mechanically complex.
The Synthes.RTM. and Kirschner-Ehmer designs are particularly susceptible to loosening under repeated cyclic loading. The Kirschner-Ehmer device relies entirely on the axial tension in the bolt to preclude pin rotation. A slight amount of loosening due to cyclic loading reduces axial tension in the bolt, allowing the pin to rotate relative to the connecting rod. The Synthes.TM. design relies on a clamp that touches the connecting rod at only two points, rendering this device susceptible to loosening.
Improved external fixator clamp designs have been described in U.S. patent application Ser. No. 08/643,512, filed May 6, 1996, entitled "External Fixator System", filed by Kraus, et al., the teachings of which are incorporated herein in their entirety by this reference. There, the clamp body had a slot that was wide enough to transversely receive and snap-fit over the connecting rod. A hook-shaped bolt, rather than the eye-bolt of the Kirschner-Ehmer designs, was used to hold the fixator pin while securing the clamp body to the rod. This configuration enabled the clamp body to be conveniently added in the middle of a rod.